Few would argue that all living species on Earth are susceptible to viruses – these microscopic parasites are ubiquitous.

But virologists have long suspected that our current view of the diversity of viruses is blinkered – all too often constrained to those causing disease in humans, animals and plants, or to those that we can grow in the laboratory.

A trip to a tropical rainforest or the African savannah gives a snapshot into the incredible diversity of visible life on Earth, but understanding the potentially mind-boggling myriad of minuscule viruses has not been so easy.

Capturing new viruses is not like netting a new species of butterfly – viruses are invisible.

Undeterred by this practical problem an international team was keen to survey invertebrates for new viral species.

Invertebrates are spineless creatures and the group includes many familiar animals, such as insects, spiders, worms and snails. They represent the vast majority of animal species in the world today.

Scientists wanting to work out the totality of viral "life" – although many virologists would argue that viruses are not truly alive – are starting to adopt techniques that reveal their genetic calling cards, revealed in the things they infect.

Image copyrightAPImage caption
Some invertebrates do carry viruses that can infect humans, but the newly identified ones probably pose very little risk

Just like powerful new telescopes are peering deeper into space, revealing a wealth of hitherto unknown stars, next-generation sequencing techniques are providing new insight into the magnitude of the invisible world of viruses; a world we call the virosphere.

We are familiar with DNA, the "stuff of life" that makes up the blueprint of our genomes. But many viruses use a different chemical to construct their genomes – a substance known as RNA.

Just like DNA, this consists of strings of individual building blocks, or bases; each designated by a different letter: A, C, G and U.

Next generation sequencing allows researchers to quickly determine the sequence of these letters. And if you work out the order of the letters on any chain of RNA, you can determine if it belongs to a virus and whether or not the virus is new.

The research team collected around 220 species of land- and water-dwelling invertebrates living in China, extracted their RNA and, using next-generation sequencing, deciphered the sequence of a staggering 6 trillion letters present in the invertebrate RNA "libraries".

When the researchers analysed this mass of data they realised that they had discovered almost 1,500 new virus species – a whopping number by any measure. Many of these were so distinct that they did not easily fit into our existing virus family tree.

Prof Elodie Ghedin from New York University, who was not directly involved with the study, told the BBC: "This is an extraordinary study providing the largest virus discovery to date.

"It will no doubt remodel our view of the virus world and redraw virus phylogeny.

"This is what happens when you combine a bold and brute force approach with the right technology and the right set of eyes."

Even though some invertebrates carry viruses that can infect humans - like zika and dengue - the study authors do not think that these newly discovered viruses pose a significant risk.

However, this cannot be ruled out entirely, and Prof Ghedin thinks that this is an important issue to address.

"If we have learned anything from these types of true discovery projects is that when we start looking into places we haven’t looked at before, we find an incredible richness that goes beyond what was suspected.

'Looking back'

The research also showed that throughout time viruses have been trading genetic material to create new species – an incredible feat according to Prof Eric Delwart from the University of California, San Francisco, who told the BBC: "It shows a lego-like ability of different viral functional units to be recombined to create new viruses even when they originate from highly divergent viruses. The plasticity of viral genomes continues to amaze."

Not only have these studies expanded our view of the diversity of viruses, they have also provided a more complete picture of virus history, as Prof Edward Holmes from the University of Sydney, who was involved in the study explained: "We have discovered that most groups of viruses that infect vertebrates – including humans, such as those that cause well-known diseases like influenza – are in fact derived from those present in invertebrates."

He also believes that his group's data shows that viruses have been infecting invertebrates for possibly billions of years, raising the prospect that invertebrates are the true hosts for many types of virus.

The researchers hope that next-generation sequencing can pave the way for virus discovery in a variety of other species. And it does not stop there.

Prof Delwart thinks that further analyses of existing next-generation datasets may yield additional virus species unlike any that we have seen before.

If future studies reveal anywhere near this number of new viruses, then we’ve only just scratched the surface. It seems that the virosphere is set to explode.

Jonathan Ball is a professor of virology at Nottingham University. This coming Saturday, he will be taking part in CrowdScience, the new BBC World Service science weekly, which starts with a question from listener Ian in Jordan which is "where did viruses come from?"